1. Technical Field of the Invention
The present invention relates to RC oscillator circuits implemented within integrated circuits and, in particular, to circuits that enable a switching of the integrated oscillator circuit to instead utilize an off-chip resistor component.
2. Description of Related Art
Resistor/capacitor (RC) oscillators of many different configurations are well know to those skilled in the art. Such oscillators are commonly built using discrete components. However, placement of these oscillators onto an integrated circuit chip is often necessary. When fully integrated (i.e., all components exist on the chip) the designer and user unfortunately lose much if not all of the ability to control oscillator operation through manipulation of either the resistance or capacitance values that drive oscillator operation. There is accordingly a need for a fully integrated oscillator design that also supports connection of an off-chip resistor.
An RC oscillator circuit implemented on an integrated circuit chip generates an oscillator output signal having a frequency set as a function of a value of an included internal resistor. The chip includes a pin to which an external resistor may be connected. A detection circuit detects the presence of the connected external resistor. Responsive to that detection, a circuit operates to connect the external resistor to the oscillator resulting in a change of the output oscillator frequency which is now set as a function of at least a value of that connected external resistor. In a preferred embodiment, the circuit operates to substitute the connected external resistor in the RC oscillator circuit.